1) Field of the Invention
The present invention relates to a driving force distribution and controlling apparatus for a vehicle and a driving force distribution and controlling method for a vehicle.
2) Description of the Related Art
Conventionally, for driving wheels of a general automobile, a differential gear is provided between the left wheel and the right wheel, and the different gear absorbs a difference in rotational speed between the left and right wheels upon turning so that it can perform turning smoothly. Further, in a four-wheel driven car, not only a differential gear for the left and right wheels but also another differential gear (center diff) for the front and rear wheels is provided in order that a difference in rotational speed between the front and rear wheels can be absorbed.
However, such conventional differential gears as described above have a characteristic that, when one of the driving wheels on one side from between the left and right wheels or between the front and rear wheels slips, driving force is not distributed to the driving wheel or wheels on the other side. Therefore, in recent years, a differential operation limiting apparatus for limiting the differential operation by a differential gear is frequently provided particularly in vehicle models designed for driving on a bad road, vehicle models designed for sports driving and so forth. As a representative example of a differential operation limiting apparatus, for example, an LSD (Limited Slip Difference: differential operation limiting apparatus) is available. By controlling the LSD electrically or mechanically, even if any driving wheel slips, driving force can be suitably distributed to suitable driving wheels.
Further, in recent years, a driving force distribution system (or a driving force distribution mechanism) has been implemented which not only operates when a driving wheel slips but also causes a driving system equipment as represented by an LSD described hereinabove to actively operate in accordance with a traveling situation of the vehicle to improve the turning performance, acceleration performance, stabilization performance and so forth of the vehicle. It is to be noted that, as particular examples included in the concept of the driving force distribution system, an electronically controlled LSD, a front-rear wheel driving force distribution mechanism capable of changing the distribution of driving force to front and rear wheels, a left-right wheel driving force distribution mechanism capable of changing the distribution of driving force to left and right wheels, and an electronically controlled coupling are available.
On the other hand, in recent years, a vehicle is equipped with an ABS (antilock brake system or antiskid brake system) so that, upon braking, the vehicle can be slowed down with certainty while the steering performance is assured.
However, in a vehicle which is equipped with both of the driving force distribution system and the ABS described above, interference sometimes occurs between control by the driving force distribution mechanism and control by the ABS.
Thus, a technique for eliminating such a trouble as just described to improve the control affinity between the driving force distribution system and the ABS has been proposed by the assignee of the present patent application (refer to Japanese Patent Laid-Open No. 2002-96651). According to the technique, the control condition of the driving force distribution system can be suitably changed over between a standard mode and another control mode (ABS-active mode) in which an effect by ABS control is not disturbed depending upon whether the ABS operates or does not operate.
However, according to the technique described above (refer to Japanese Patent Laid-Open No. 2002-96651), when the driving force distribution system is in the ABS-active mode in this manner, an effect by the driving force distribution system which has been available before the ABS is rendered operative sometimes drops.
Further, where the road friction p is high, since the difference between the effect by the standard mode and the effect by the ABS-active mode is great, for example, if the control condition of the driving force distribution system is changed over from the standard mode to the ABS-active mode (or reversely from the ABS-active mode to the standard mode) while the vehicle is traveling on a high-p road such as, for example, a dry asphalt road, then a shock arising from the mode changeover is generated, and there is the possibility that an unfamiliar feeling to the driving may be provided to the driver.
Therefore, it is demanded for a vehicle which is equipped with both of a driving force distribution mechanism and an ABS apparatus that, even if the ABS is rendered operative, the behavior of the vehicle can be stabilized while the turning performance, acceleration performance, stabilization performance and so forth are improved by the driving force distribution mechanism.
On the other hand, if such a failure occurs that a signal line for interconnecting a controller of the driving force distribution system or a controller of the ABS is disconnected or short-circuited, then there is the possibility that such a situation may occur that, although the ABS is operating, the driving force distribution system may operate in the standard mode. In such an instance, there is the possibility that the control by the driving force distribution system may disturb the effect by the ABS. Further, if the failure described above gives rise to a wrong decision of the driving force distribution system that the ABS is operating although the ABS is not actually operating, then the driving force distribution cannot be performed correctly.